(a) Field of the Invention
This invention relates to an improved process for the preparation of indole from aniline and ethylene glycol.
(b) Description of the Invention
Indole has found wide-spread commercial utility as a raw material for the chemical industry, for example, for perfumes and dyestuffs. Its application as a raw material for the synthesis of amino acids has attracted special attention in recent years. Indole has conventionally been prepared from expensive raw materials or through long and complicated preparation steps. As an economical process for preparing indole from inexpensive raw materials and through simpler steps, the vapor-phase catalytic reaction of aniline and ethylene glycol is known.
As catalysts useful for the above reaction, a variety of solid acid catalysts and metallic catalysts have been proposed. The principal reaction product of the vapor-phase catalytic reaction is of course indole. However, various compounds, in addition to indole, are also formed as a result of side reactions although their yields are not high. Moreover, it is also known that excess amounts of aniline in the reaction system are required to obtain indole with a good yield. When indole is industrially prepared in accordance with the above-mentioned process, it may not be possible to obtain it with sufficient purity as a raw material for the chemical industry unless a large amount of aniline which is contained in the resulting reaction mixture is separated and recovered for reutilization and unless the indole is purified by a suitable method such as distillation. Distillation is generally used as a method for effecting the separation and purification of indole on an industrial scale.
However, it is difficult to obtain indole with sufficient purity to permit its use as a raw material for the chemical industry if the recovery of the accompanying aniline and the purification of the indole are effected by ordinary distillation, because in ordinary distillation, aniline in an amount far greater than that expected from the gas-liquid equilibrium of aniline and indole is allowed to mix in the intended product, and the concentration of the thus-mixed aniline cannot be lowered even if the reflux ratio or theoretical plate number of a rectification column is increased.